Add Dangerfile (#4)

Author: KS1019

Dangerfile.swift

@@ -0,0 +1,14 @@
+// swiftlint:disable line_length
+import Danger
+
+let danger = Danger()
+let allSourceFiles = danger.git.modifiedFiles + danger.git.createdFiles
+
+if allSourceFiles.first(where: { $0.fileType == .swift }) != nil {
+    // Only modified and created files are linted.
+    // See https://github.com/danger/swift/blob/d951a755f7e9335321160272698a2ede9dc572a3/Sources/Danger/Plugins/SwiftLint/SwiftLint.swift#L91
+    let violation = SwiftLint.lint()
+    if violation.isEmpty {
+        message("No violation found!!!")
+    }
+}